This invention relates to an apparatus for reproducing a digital signal from a digital audio recording medium (a disk, a tape or the like), and more particularly to a digital signal reproucing circuit suitable for use in a system where the transmission speed of a digital signal is variable.
A compact disk player (referred to hereinafter as a CD player) using a compact disk (a CD) is now popular as a home-use digital signal reproducing apparatus which can reproduce a high-quality music signal from the disk. Very small concavities called pits are engraved on this disk to record a digital signal, and, when the disk is rotated in the playback mode, an optical pickup detects the pits to reproduce the digital signal recorded on the disk. After processing the reproduced digital signal for error correction, etc., the digital signal is applied to a digital-analog converter (a DAC) to reproduce the original music signal.
In order that the CD player can accurately reproduce the music signal, a signal processing circuit is required which controls the CD player so as to rotate the disk at a predetermined rotation speed based on a fixed frequency. The processing circuit is also required to reproduce a clock signal synchronous with the digital signal reproduced from the disk so as to fetch data, and, after processing including error correction, delivers the data sampled at a sampling frequency based on the fixed frequency.
A prior art signal processing circuit of this kind is disclosed in, for example, JP-A-58-219852 and JP-A-59-124012. According to the disclosed circuit, the rotation control of a disk and the signal processing for the output of data are performed on the basis of an oscillation frequency of a single crystal oscillator, and a clock signal for data fetching purpose is produced by a PLL circuit to be synchronized with the data. Such a signal processing circuit carries out its processing operation normally in an ordinary playback mode. However, no consideration has been taken for the reproduction of a music signal at a variable speed, for example, double speed, as when the disk is rotated at double speed (a speed two times as high as that in the ordinary playback mode) for the purpose of dubbing of the reproduced music signal on a cassette tape run at double speed.
In a digital audio tape recorder (a DAT recorder) using a magnetic tape, the magnetic tape 80 is wound over an angle of about 90.degree. around part of a rotary cylinder 81 having a diameter a as shown in (A) of FIG. 9, and a digital signal is reproduced from the magnetic tape 80 by rotary magnetic heads 82A and 82B provided on the rotary cylinder 81. Since, in this case, the rotary magnetic heads 82A and 82B are disposed at an angular interval of 180.degree. therebetween on the cylinder 81, the digital signal is intermittently reproduced as shown in (A) of FIG. 10. In such a case, the period of signal presence is equal to the period of signal absence.
On the other hand, in the case of, for example, high-speed dubbing of such a magnetic tape 80, the magnetic tape 80 is run at a high speed, and the speed of scanning with the rotary magnetic heads is also increased. Suppose now that the magnetic tape 80 is wound over an angle of about 45.degree. around part of a rotary cylinder 83 having a diameter 2a as shown in (B) of FIG. 9 and is scanned for reproduction by rotary magnetic heads 84A and 84B provided at an angular interval of 180.degree. therebetween on the cylinder 83. When the rotation speed of the cylinder 83 is selected to be equal to that of the cylinder 81 shown in (A) of FIG. 9, the digital signal is intermittently reproduced as shown in (B) of FIG. 10. In this case, the period of signal presence is 1/3 of the period of signal absence, and this period of signal presence is 1/2 of that shown in (A) of FIG. 10.
That is, when it is supposed that the playback mode shown in (A) of FIG. 9 is an ordinary playback mode, the playback mode shown in (B) of FIG. 9 is a double-speed playback mode. This is because, in the playback mode shown in (B) of FIG. 9, the rotary heads 84A and 84B rotate at a speed two times as high as that of the rotary heads 82A and 82B in the playback mode shown in (A) of FIG. 9. (The traveling speed of the magnetic tape 80 in this case is two times as high as that in the ordinary playback mode.) It will be seen from (A) and (B) of FIG. 10 that the time base of the reproduced digital signal in the case of the double-speed playback mode is compressed to 1/2 of that of the ordinary playback mode, and, therefore, the transmission speed is doubled in the former case.
The prior art signal processing circuit may be able to produce a clock signal synchronous with the digital signal intermittently reproduced in the ordinary playback mode. However, in the prior art signal processing circuit, the oscillation frequency of the single crystal oscillator is used as the basic frequency, and the clock signal is produced by the PLL circuit. Therefore, when the playback mode is switched over to the double-speed playback mode in which the transmission speed of the reproduced digital signal differs greatly from that in the ordinary playback mode, the PLL circuit cannot follow up the reproduced digital signal. As a result, a clock signal synchronized with the reproduced digital signal cannot be obtained, and processing including error correction of the reproduced digital signal cannot be achieved.